The present invention relates to an impedance detection circuit and an impedance detection method, and more specifically, to an impedance detection circuit capable of removing an influence of a parasitic capacitor, and an impedance detection method.
In recent years, with sophistication of systems of electronic devices etc., circuits for detecting impedances, such as a capacitance sensor, are being widely used. Therefore, there is desired an impedance detection circuit that is small-sized, easy to be adjusted, and further can detect an impedance correctly without being affected by an outer environment so as to be able to be used being mounted on various places.
FIG. 8 is a circuit diagram for explaining the impedance detection circuit disclosed in Japanese Unexamined Patent Application Publication No. 2002-350477. An impedance detection circuit according to Japanese Unexamined Patent Application Publication No. 2002-350477 shown in FIG. 8 operates to convert an unknown electrostatic capacitance Cs of a capacitor 130 to be measured (a measured capacitor 130) to a corresponding voltage (i.e., to a voltage proportional to the Cs). In FIG. 8, the impedance detection circuit is composed of a detection circuit 131, a phase compensating circuit 132, an amplitude compensating circuit 133, a subtraction circuit 134, and an alternating current (AC) signal generator 135.
The detection circuit 131 is a circuit for detecting the capacitance Cs of the measured capacitor 130. The detection circuit 131 has an operational amplifier 141 whose voltage gain is very much larger than its closed loop gain, seemingly almost infinite. A feedback resistor 142 is coupled between an output terminal and an inverting input terminal (−) of the operational amplifier 141, and negative feedback is given to the operational amplifier 141. A driving signal outputted from the AC signal generator 135 is impressed to a non-inverting input terminal (+) of the operational amplifier 141. An other end of a signal line 143 whose one end is coupled with one electrode of the measured capacitor 130 is coupled to the inverting input terminal (−) of the operational amplifier 141. An other electrode of the measured capacitor 130 is grounded, is added a fixed direct current (DC) bias Vh, or is not grounded.
In order to prevent undesired signals, such as noises from the outside, from being induced to the signal line 143, the circumference of the signal line 143 is surrounded by a shielding wire 144. The shielding wire 144 is not grounded but is coupled with the non-inverting input terminal (+) of the operational amplifier 141.
Incidentally, a symbol Cp of FIG. 8 designates the parasitic capacitor produced in a portion where the signal line 143 is not shielded, i.e., a portion where the signal line is exposed, and there is a possibility that a surrounding AC signal is impressed to the inverting input terminal (−) through this.
Since the operational amplifier 141 is given the negative feedback through the feedback resistor 142 and the voltage gain of the operational amplifier 141 is very much larger than a closed loop gain, and is seen almost infinite, both input terminals of the operational amplifier 141 are in an imaginary short state. That is, a potential difference between the inverting input terminal (−) and the non-inverting input terminal (+) of the operational amplifier 141 is substantially zero. Therefore, since the signal line 143 and the shielding wire 144 are the same potential (so-called voltage), a stray capacitance (the parasitic capacitor) produced between the signal line 143 and the shielding wire 144 can be cancelled. This fact holds regardless of the length of the signal line 143 and further regardless of movement, bending, etc. of the signal line 143.
The phase compensating circuit 132 is a circuit for compensating the phase of the driving signal outputted from the AC signal generator 135. The phase compensating circuit 132 includes an operational amplifier 151, and a feedback resistor 152 of resistance R1 is coupled between an output terminal and an inverting input terminal (−) of the operational amplifier 151. The driving signal outputted from the AC signal generator 135 is supplied to the inverting input terminal (−) of the operational amplifier 151 through a resistor 153 of resistance R1. Moreover, this driving signal is also supplied to a non-inverting input terminal (+) of the operational amplifier 151 through a variable resistor 154 of resistance Ri2. This non-inverting input terminal (+) is grounded through a capacitor 155 of capacitance C2.
The amplitude compensating circuit 133 is a circuit for compensating the amplitude of the driving signal from the AC signal generator 135. The amplitude compensating circuit 133 includes an operational amplifier 161, and a feedback resistor 162 of resistance Rf3 is coupled between an output terminal and an inverting input terminal (−) of the operational amplifier 161. An output voltage of the phase compensating circuit 132, i.e., an output voltage V2 of the operational amplifier 151 is impressed to the inverting input terminal (−) of the operational amplifier 161 through a resistor 163 of resistance Ri3, and the non-inverting input terminal (+) is grounded. Further, an output V3 of the operational amplifier 161 is supplied to the inverting input terminal (−) of the operational amplifier 141 through a resistor 145 of resistance Ri1. Additionally, although a current produced by the electrostatic capacitance Cs and a parasitic capacitor Cp flows toward the feedback resistor 142. At this time, the adjusted current is flowing into the feedback resistor 142 from an output terminal of the amplitude compensating circuit 133 through the resistor 145 so as to cancel the current corresponding to the added Cp.
The subtraction circuit 134 is a circuit for subtracting an output of the AC signal generator 135 from an output of the detection circuit 131. The subtraction circuit 134 includes an operational amplifier 171, and a feedback resistor 172 of resistance R1 is coupled between an output terminal and an inverting input terminal (−) of the operational amplifier 171. The output of the detection circuit 131, i.e., an output V1 of the operational amplifier 141 is impressed to the inverting input terminal (−) of the operational amplifier 171 through a resistor 173 of resistance R1; the output of the AC signal generator 135 is impressed to the non-inverting input terminal (+) through a resistor 174 of resistance R1. A DC bias Vh is impressed to this non-inverting input terminal (+) through a resistor 175 of resistance R1.
Although the driving signal outputted from the AC signal generator 135 is a sum of an AC signal Vdv and the DC bias Vh, the DC bias Vh may be zero. Below, an operation of the impedance detection circuit shown in FIG. 8 will be explained. The signal line 143 that couples the inverting input terminal of the operational amplifier 141 of the detection circuit 131 and the one of electrodes forming the measured capacitor 130 is surrounded and shielded by the shielding wire 144. However, even when the measured capacitor 130 is not coupled, an output of this impedance detection circuit, i.e., an output signal Vout of the operational amplifier 171 is shifted from the phase of the AC signal Vdv although it is small. This is because the parasitic capacitor Cp produced in a portion where the signal line 143 is not shielded remains.
Therefore, in order to cancel such a phase shift, after adjusting the phase and amplitude of the AC signal Vdv with the phase compensating circuit 132 and the amplitude compensating circuit 133, respectively, the signal after the adjustment, i.e., the output V3 of the operational amplifier 161 is fed back to the inverting input terminal of the operational amplifier 141 of the detection circuit 131 through the resistor 145.
In this case, denoting an angular frequency of the AC signal Vdv by ω, an output V2 of the phase compensating circuit 132 is expressed as follows.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                      V          2                =                                                            1                -                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                                                                                  1                +                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                                                                            ⁢                          V              dV                                +                      V            h                                              (        1        )            
The output V3 of the amplitude compensating circuit 133 is expressed as follows.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                                      V          3                =                              -                                                                                R                                          f                      ⁢                                                                                          ⁢                      3                                                        ⁢                  1                                -                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                                                                                                                        R                                          i                      ⁢                                                                                          ⁢                      3                                                        ⁢                  1                                +                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                                                                                    ⁢                      (                                          V                dV                            +                              V                h                                      )                                              (        2        )            
The output V1 of the detection circuit 131 is expressed as follows.
                    [                  Equation          ⁢                                          ⁢          3                ]                                                                      V          1                =                              j            ⁢                                                  ⁢            ω            ⁢                                                  ⁢                          C              s                        ⁢                          R                              f                ⁢                                                                  ⁢                1                                      ⁢                          V              dV                                +                      j            ⁢                                                  ⁢            ω            ⁢                                                  ⁢                          C              p                        ⁢                                          R                                  f                  ⁢                                                                          ⁢                  1                                            ⁡                              (                                                      V                    dV                                    +                                      V                    h                                                  )                                              +                                    {                              1                +                                                      R                                          f                      ⁢                                                                                          ⁢                      1                                                                            R                                          i                      ⁢                                                                                          ⁢                      1                                                                      +                                                                            R                                              f                        ⁢                                                                                                  ⁢                        1                                                                                    R                                              i                        ⁢                                                                                                  ⁢                        1                                                                              ⁢                                                            R                                              f                        ⁢                                                                                                  ⁢                        3                                                                                    R                                              i                        ⁢                                                                                                  ⁢                        3                                                                              ⁢                                                            1                      -                                              j                        ⁢                                                                                                  ⁢                        ω                        ⁢                                                                                                  ⁢                                                  C                          2                                                ⁢                                                  R                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                                      1                      +                                              j                        ⁢                                                                                                  ⁢                        ω                        ⁢                                                                                                  ⁢                                                  C                          2                                                ⁢                                                  R                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                                                          }                        ⁢                          (                                                V                  dv                                +                                  V                  h                                            )                                                          (        3        )            
The output signal Vout of the subtraction circuit 134 is expressed as follows.
                    [                  Equation          ⁢                                          ⁢          4                ]                                                                                                                V                out                            =                            ⁢                                                                    -                    j                                    ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    s                                    ⁢                                      R                                          f                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      V                    dv                                                  -                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    p                                    ⁢                                                            R                                              f                        ⁢                                                                                                  ⁢                        1                                                              ⁡                                          (                                                                        V                          dv                                                +                                                  V                          h                                                                    )                                                                      -                                                                                                      ⁢                                                                                          R                                              f                        ⁢                                                                                                  ⁢                        1                                                                                    R                                              i                        ⁢                                                                                                  ⁢                        1                                                                              ⁢                                      {                                          1                      +                                                                                                    R                                                          f                              ⁢                                                                                                                          ⁢                              3                                                                                                            R                                                          i                              ⁢                                                                                                                          ⁢                              3                                                                                                      ⁢                                                                              1                            -                                                          j                              ⁢                                                                                                                          ⁢                              ω                              ⁢                                                                                                                          ⁢                                                              C                                2                                                            ⁢                                                              R                                                                  i                                  ⁢                                                                                                                                          ⁢                                  2                                                                                                                                                                          1                            +                                                          j                              ⁢                                                                                                                          ⁢                              ω                              ⁢                                                                                                                          ⁢                                                              C                                2                                                            ⁢                                                              R                                                                  i                                  ⁢                                                                                                                                          ⁢                                  2                                                                                                                                                                                                          }                                    ⁢                                      (                                                                  V                        dv                                            +                                              V                        h                                                              )                                                  +                                  V                  h                                                                                                        =                            ⁢                                                                    -                    j                                    ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                                      C                    s                                    ⁢                                      R                                          f                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      V                    dv                                                  -                                                      (                                          P                      +                      Qj                                        )                                    ⁢                                      V                    dv                                                  -                                                      (                                          P                      -                      1                      +                      Qj                                        )                                    ⁢                                                            V                      h                                        .                                                                                                          (        4        )            
Here, P and Q are as follow.
                    [                  Equation          ⁢                                          ⁢          5                ]                                                            P        =                                            R                              f                ⁢                                                                  ⁢                1                                                    R                              i                ⁢                                                                  ⁢                1                                              +                                                    R                                  f                  ⁢                                                                          ⁢                  1                                                            R                                  i                  ⁢                                                                          ⁢                  1                                                      ⁢                                          R                                  f                  ⁢                                                                          ⁢                  3                                                            R                                  i                  ⁢                                                                          ⁢                  3                                                      ⁢                                          1                -                                                      ω                    2                                    ⁢                                      C                    2                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                        2                                                                              1                +                                                      ω                    2                                    ⁢                                      C                    2                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                        2                                                                                                          (        5        )                                [                  Equation          ⁢                                          ⁢          6                ]                                                            Q        =                              ω            ⁢                                                  ⁢                          C              p                        ⁢                          R                              f                ⁢                                                                  ⁢                1                                              -                                                    R                                  f                  ⁢                                                                          ⁢                  1                                                            R                                  i                  ⁢                                                                          ⁢                  1                                                      ⁢                                          R                                  f                  ⁢                                                                          ⁢                  3                                                            R                                  i                  ⁢                                                                          ⁢                  3                                                      ⁢                                          2                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  C                  2                                ⁢                                  R                                      i                    ⁢                                                                                  ⁢                    2                                                                              1                +                                                      ω                    2                                    ⁢                                                                          ⁢                                      C                    2                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      2                                        2                                                                                                          (        6        )            
Therefore, it is possible to realize conditions of P=0 and Q=0 by adjusting the resistance Ri2 of the resistor 154, and the resistance Ri3 of the resistor 163 depending on the parasitic capacitor Cp. In measuring the electrostatic capacitance Cs of the measured capacitor 130, an adjustment of these resistances Ri2 and Ri3 is performed before coupling the measured capacitor 130. Under this condition, the resistance Ri2 of the resistor 154 and the resistance Ri3 of the resistor 163 are adjusted so as to fulfill the following equation.
                    [                  Equation          ⁢                                          ⁢          7                ]                                                                      R                      i            ⁢                                                  ⁢            2                          =                              1            +                                          1                +                                                      ω                    2                                    ⁢                                      C                    p                    2                                    ⁢                                      R                                          i                      ⁢                                                                                          ⁢                      3                                        2                                                                                                          ω              2                        ⁢                          C              2                        ⁢                          C              p                        ⁢                          R                              i                ⁢                                                                  ⁢                3                                                                        (        7        )                                [                  Equation          ⁢                                          ⁢          8                ]                                                                      R                      f            ⁢                                                  ⁢            3                          =                              R                          i              ⁢                                                          ⁢              3                                ⁢                                    1              +                                                ω                  2                                ⁢                                  C                  p                  2                                ⁢                                  R                                      i                    ⁢                                                                                  ⁢                    1                                    2                                                                                        (        8        )            
As a result, the output signal Vout of the subtraction circuit 134 becomes as follows.[Equation 9]Vout=jωCsRf1VdV+Vh  (9)
This equation (9) indicates that the amplitude of the output signal Vout of the subtraction circuit 134 is proportional to the electrostatic capacitance Cs of the measured capacitor 130 without being affected by the parasitic capacitor Cp.
As explained above, in the impedance detection circuit shown in FIG. 8, adjusting the values of the two variable resistors enables to obtain an output signal having a value proportional to the electrostatic capacitance Cs of the measured capacitor 130 without being affected by the parasitic capacitor Cp.